The present invention relates generally to diagnostic imaging and, more particularly, to a system and method for dynamic spectral filtration for providing an increased separation of mean energies between low and high kVp image acquisitions.
Medical imaging devices comprise x-ray systems, magnetic resonance (MR) systems, ultrasound systems, computed tomography (CT) systems, positron emission tomography (PET) systems, ultrasound, nuclear medicine, and other types of imaging systems. Typically, in CT imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry opening within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction. Such typical systems, however, do not include an ability to discriminate spectral energy content of x-rays as they pass through an object being imaged.
However, as known in the art, dual or multi-energy spectral CT systems have been developed that can reveal the densities of different materials in an object and generate images acquired at multiple monochromatic x-ray energy levels. In the absence of object scatter, a system derives the behavior at a different energy based on a signal from two regions of photon energy in the spectrum: the low-energy and the high-energy portions of the incident x-ray spectrum. In a given energy region of medical CT, two physical processes dominate the x-ray attenuation: (1) Compton scatter and the (2) photoelectric effect. The detected signals from two energy regions provide sufficient information to resolve the energy dependence of the material being imaged and the relative composition of an object composed of two hypothetical materials.
Different approaches have been developed to realize dual energy or spectral imaging. To name a few, dual x-ray source and detector, a single x-ray source with an energy discriminative detector, and a single x-ray source and detector with multiple acquisitions at different kVp or interleaved with fast kVp switching capability are examples of techniques.
In a dual x-ray source and detector system, typically two x-ray sources are provided, each having a respective detector positioned opposite thereto such that x-rays may be emitted from each source having a different spectral energy content. Thus, based on the known energy difference of the sources, a scintillating or energy integrating device may suffice to distinguish energy content and different materials within the object being imaged.
In a single x-ray source with an energy discriminative detector, energy sensitive detectors may be used such that each x-ray photon reaching the detector is recorded with its photon energy. Such systems may use a direct conversion detector material in lieu of a scintillator.
In a single x-ray source and detector arrangement, a conventional third generation CT system may acquire projections sequentially at different peak kilovoltage (kVp) levels, which changes the peak and spectrum of energy of the incident photons comprising the emitted x-ray beams. Two scans are acquired—either (1) back-to-back sequentially in time where the scans require two rotations around the subject (i.e., rotate-rotate), or (2) interleaved as a function of the rotation angle requiring one rotation around the subject, in which the tube operates at, for instance, 80 kVp and 140 kVp potentials.
When dual energy data is acquired in a rotate-rotate fashion, data is collected at a high kV setting over 180 degrees plus ½ the fan angle of the x-ray beam, so as to cover typically about 210 degrees. A short time is then taken to change the kV setting, with around 100 degrees of gantry rotation occurring during this changing (depending on the gantry rotation speed). The second kV dataset is then collected over 180 degrees plus ½ fan angle, so as to again cover typically about 210 degrees. Dual energy data is thus collected in rotate-rotate in less than two full rotations.
When dual energy data is acquired in an interleaved fashion, an input voltage to the x-ray source is switched quickly between the low and high kVp potentials so that a full dataset is collected in 1 rotation, which allows a close correlation between imaging data sets. The high and low kV datasets are thus being continuously collected in a rapid alternating fashion around the patient, with each rotation being divided into approximately 1000 views, so about 350 degrees/1000 views ˜0.35 deg/view for each energy level, for example. While acquisition in such an interleaved fashion advantageously provides close temporal registration between the 2 kV datasets so as to minimize artifacts due to patient motion, a drawback to the interleaved acquisition is that, because the switching occurs very rapidly on a single x-ray source, there is little opportunity to change the filtration between the two samples. As a result, there is a spectral (energy) overlap between the two samples that inherently limits the amount of energy separation between them. As known in the art, it is desirable to increase energy separation between low and high kVp operation in order to increase the energy separation of the high and low kV spectrums so as to yield higher dual energy contrast (i.e., increase the contrast between the two materials being evaluated—the “dual energy (DE) ratio”). However, it is not feasible to simply decrease the low kVp or increase the high kVp in order to increase energy separation therebetween. Lowering the low kVp may have limited signal-to-noise and cause other limitations in image reconstruction, with most x-rays get absorbed at lower kV settings (i.e., higher absorbed dose) so as to not make it to the detector. Increasing the high kVp may cause system instability and spit activity and may cause other limitations in system operation, with higher kV also yielding less contrast since fewer x-rays interact with object being imaged.
Therefore, it would be desirable to have a system and method of increasing energy separation in dual energy CT.